Summary

In a confluent culture of WI-38 cells the membrane area available for nutrient uptake is greatly reduced and the possibility exists that this reduction in uptake capacity of the cell is a contributory factor in contact inhibition. Insulin has been reported by many authors to facilitate glucose uptake and also to stimulate protein, DNA and RNA synthesis, glycolysis, pino-cytosis and growth in cultured cells. The effect of insulin on WI-38 cells was determined, therefore, to find out whether it enabled the cell to escape from contact inhibition of growth.

The action of insulin was found to be dependent upon medium composition. Growth and protein synthesis were stimulated in Eagle's minimal essential medium, but not when this medium was supplemented with glucose and glutamine. Apparently insulin is only effective when high-energy compounds become limiting. Whilst insulin did not induce any post-confluent division, the protein content of cells was increased by 30%, and this was correlated with an increased rate of protein synthesis. Despite this increased activity in protein metabolism, the utilization of amino acids was less in the presence of insulin indicating that a control mechanism for more economical utilization of amino acids for protein synthesis was activated by insulin. Insulin had no effect on RNA synthesis, and only a slight inhibitory effect on DNA synthesis. Evidence was produced suggesting that insulin blocked cell division and encouraged differentiation. Glucose uptake and incorporation into the cell was stimulated by insulin, and this was especially noticeable after the cell sheet became confluent. The turnover of labelled glucose and derivatives was also enhanced by insulin and this was accompanied by a much higher rate of lactic acid production.

It is concluded that insulin does not overcome contact inhibition and permit post-confluent division, but that it does enable the cell to take up and utilize nutrients more efficiently in confluent cultures with a resultant increase in metabolic activity and cell size.